It is desirable to control gas discharge devices to assure accurate and consistent output. For example, gas discharge lamps are very useful for photographic enlargers since they can be formed to any size and shape to uniformly illuminate a negative and they allow excellent choices of appropriate light wavelengths. Such sources have not become popular for that purpose since the available power supplies allow the light output to drift. Thus a better control is needed, although gas discharge loads present special control problems.
When an AC source applies voltage to a gas discharge lamp the gas is not ionized at the beginning of each half-cycle and has an extremely high impedance. When the voltage becomes high enough to ionize the gas, current flows and light is emitted at a level proportional to the current. The ionized gas presents a low impedance load and the voltage necessary to sustain the discharge is much lower than the voltage required to initiate the discharge. Indeed, if the higher voltage is maintained after ionization, the current through the small impedance becomes large enough to destroy the lamp. As the voltage drops at the end of the half-cycle the discharge is extinguished and the process repeats in each half-cycle. These non-linearities put peculiar demands on the power source and uniform control of the light output is difficult to attain with prior power supplies.
The standard device for supplying current limited power to a gas discharge tube is a high leakage reactance transformer. Such a reactance transformer, as known in the prior art, is shown in FIG. 1. A three-legged magnetic core 10 has a first leg 12 containing an air gap 13 and has a specific constant magnetic reluctance determined by the size of the gap. A second leg 14 comprising a center leg has a primary winding 16 for connection to an AC supply. The third leg 18 has a secondary winding 20 for connection to the gas discharge load. The flux path through the second and third legs links the primary and secondary windings and forms a first loop A. The flux path through the second and first legs forms a second loop B.
The loop A has a variable reluctance dependent on the secondary load while the loop B has a fixed reluctance which is much greater than the unloaded loop A. At low load, the loop A will have most of the flux flowing through it and the secondary voltage will be high. As the load increases, the reluctance of loop A increases and the secondary voltage decreases. As the load on the secondary winding approaches an electrical short, the majority of the flux flows through the loop B because its fixed reluctance is now lower than the high reluctance of loop A. Thus at low secondary voltage the current is high and is limited to a value set by the fixed reluctance loop B.
When the high leakage reactance transformer drives a gas discharge lamp, a waveform like that of FIG. 2 is generated. The sinusoidal AC open circuit voltage is shown in dotted lines. The voltage across the lamp reaches the ionization voltage C and then drops to level D when secondary current flows at a value determined by the current limiting transformer. The current is shown on the same graph. When the voltage drops below the discharge sustaining level E the current is extinguished. Then the process is repeated in the next half-cycle. Since the light output is proportional to lamp current, it is essentially constant during the periods of current conduction. The overall light output is subject to drifting and moreover there is a need for light level adjustment to dim the illumination when desired.
Several prior proposals have addressed the illumination control problem as shown in the following U.S. patents. Spreadbury U.S. Pat. No. 4,350,934 shows an inductive reactance for discharge device control having a gap in one leg and a control winding on that leg for varying the value of the reactance to control the supply current or the power consumed by the device. Increased current in the control winding increases the reluctance of that leg to increase the current limit in the other leg. The supply current to the device thus can be varied between a limit set by the gap and an unlimited destructive value. The transformer cannot be controlled to diminish the current below the value set by the gap. Elms U.S. Pat. No. 4,162,429 shows a variable reactor for discharge lamp control having a core similar to that of FIG. 1 herein except that each leg contains a gap. A separate starter winding and a specific control circuit are provided. Owens U.S. Pat. No. 4,562,384 shows a discharge lamp ballast having a core with a pair of shunts and windings only on the leg of the core containing the gap. Additional controls for transformer supplied discharge lamps are revealed by Davenport U.S. Pat. No. 4,384,239 and Agarwala U.S. Pat. No. 4,350,933.